How to add a RGB Curve node
As mentioned in the previous post, ideally you fix the input file because for whatever reasons the object was imaged under poor lighting conditions or the software used might have a mode to adjust the brightness of the polygon/vertex colours and it was set to darken. 

Regaining the fidelty / vibrance of the colours is tricky and the techniques used might produce unsatisfactory results. 

One fast way for Blender Internal is to use Ambient Occlusion and Environment Lighting, from the World tab. You'll  notice the result is a little blown out (as if over exposed) if the guy is wearing a light pink shirt.



Blender Internal's materials also has a Nodes mode, and it supports Vertex Colors. It allows you to adjust the colours with an RGB Curve too, and you don't need to enable Ambient Occlusion or environment lighting



Or maybe a hue / saturation node:





Anything you do to make it render nicely won't be a true representation of what the output will look like if it's 3d printed (I think you are doing this to render product shots?) -- unless you somehow modify the Vertex Color map.

You can bake to Vertex Colors using the Bake feature set to 'Full Render'. I would create another Vertex Color layer and set that to active in the panel, before doing a bake. This will prevent bake from over-riding your original.



bake: